Field of the Invention
The present invention relates to a semiconductor device using a novel compound semiconductor material and, more particularly, to a semiconductor laser capable of light emission up to the ultraviolet region and a method of manufacturing the same.
Description of the Related Art
In order to construct a high-speed and high-density information processing system, realization of a shortwave semiconductor laser has been desired. To realize the shortwave semiconductor laser, a semiconductor material is required to have properties of the following three items.
1) The material must be a direct transition type. PA1 2) The material must allow formation of a p-n junction. PA1 3) The material must allow formation of a heterojunction.
A conventionally known example of a semiconductor material which satisfies all of the above three conditions is InGaAlP. When this InGaAlP-based material is used, it is possible to obtain laser oscillation of up to 580 nm.
To obtain a semiconductor laser having a shorter wavelength, a direct transition type Group II-VI nitride having a band gap of 3 eV or more, for example, has been examined as a promising material. However, since conductivity control has not been achieved yet by using the Group II-VI nitride, no successful use of this nitride has been reported. In particular, almost no examination has been made on a semiconductor laser material, which oscillates in the ultraviolet region having a shorter wavelength than 300 nm and therefore can be used in a wide variety of applications such as an excitation light source for various phosphors and a light source for a photochemical reaction, regardless of a strong demand for it.
The emission energy of an ultraviolet semiconductor laser is 5 eV or more which is close to the chemical bond energy of a laser material. In this respect, the ultraviolet semiconductor laser is required to have characteristics different from those of conventional laser materials from near-infrared to visible regions. That is, the crystal structure of the ultraviolet semiconductor laser must not be damaged by a photon energy upon light emission. For this purpose, a crystal is desired to have a strong lattice with small ionicity as well as a wide band gap. Importance of this condition is apparent also from an often observed phenomenon in that an alkali halide which is an ionic crystal having a wide band gap causes a defect called a color center and is colored upon by a radiation of ultraviolet light.
Examples of a semiconductor material having a band gap of 5 eV or more and capable of allowing conductivity control are limited to only a few materials such as a GaAlN-based material, cubic c-BN, and diamond. Of these materials, although c-BN and diamond have band gaps of 6 eV or more, it is very difficult to synthesize them because they are easily formed into graphite-like substances having an SP.sup.2 configuration. In addition, each of the two materials is of an indirect transition type, and no proper material for constituting a heterojunction in combination with it has been found. On the other hand, the GaAlN-based material is of a direct transition type. However, this material is limited to a mixed crystal having a considerably high Al content and easily causes crystal defects because the material is a highly ionic crystal with a weak bond, and it is very difficult to perform conductivity control of the material. In addition, the GaAlN-based material has a high possibility of causing defects upon light emission of a high energy.
As described above, there has been almost no suggestion of even a possibility of a semiconductor material required to realize a semiconductor laser capable of light emission in the ultraviolet region, i.e., having a satisfactorily wide band gap of 4 to 5 eV, capable of allowing p-n control, and being sufficiently strong so as not to be damaged by the energy of an emission wavelength.